The A1C test measures the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it. A small blood sample is all it takes, and unlike a fasting glucose test, you don’t need to skip meals or prepare in any special way beforehand. The result reflects your average blood sugar over roughly the past two to three months, giving a much broader picture than a single glucose reading can.
What the Test Actually Measures
Glucose in your bloodstream naturally sticks to hemoglobin as blood circulates through your body. The higher your blood sugar runs over time, the more hemoglobin gets coated. Since red blood cells live about three months before your body replaces them, the amount of glucose stuck to hemoglobin at any given moment acts as a rolling average of your blood sugar during that window.
Your result comes back as a percentage. An A1C of 6%, for example, means that 6% of your hemoglobin has glucose attached. There’s also a formula that converts the percentage into an estimated average glucose (eAG) in the same units a glucose meter would show: multiply your A1C by 28.7, then subtract 46.7. So an A1C of 7% translates to an estimated average blood sugar of about 154 mg/dL.
How the Blood Sample Is Analyzed
Labs use two main approaches to measure A1C. One is a technique called high-performance liquid chromatography (HPLC), which physically separates different forms of hemoglobin and measures how much of the glycated (glucose-coated) form is present. The other common method is an immunoassay, which uses antibodies designed to latch onto glycated hemoglobin specifically. Both methods are fast, with some analyzers producing a result in just over a minute.
To make sure results are consistent no matter which lab or method is used, a certification program called the NGSP standardizes all A1C tests against the same reference. This ensures your result at one clinic can be meaningfully compared to a result from another, and that it aligns with the landmark studies that originally linked specific A1C levels to diabetes complications.
What the Numbers Mean
The American Diabetes Association sets three ranges for nonpregnant adults:
- Below 5.7%: Normal blood sugar regulation.
- 5.7% to 6.4%: Prediabetes. Blood sugar is running higher than normal but hasn’t reached the diabetes threshold.
- 6.5% or higher: Diabetes, when confirmed by a repeat test or a second type of blood sugar test.
These cutoffs aren’t arbitrary. They’re drawn from large studies showing where the risk of diabetes-related complications begins to climb meaningfully.
How Often You’ll Get Tested
If your A1C is within your target range and your treatment plan hasn’t changed, testing every six months is typical. If your medications have been adjusted recently or your last result was above goal, expect testing every three months. Because the test reflects a roughly 90-day window, testing more frequently than that wouldn’t capture meaningful new information.
At-Home and Point-of-Care Tests
You can buy finger-stick A1C kits or get tested at a pharmacy without a full lab visit. These point-of-care devices are convenient, but they come with a tradeoff in precision. A 2017 review of 61 studies found that all 13 point-of-care devices examined showed measurable bias compared to lab results. Nine of the 13 tended to read lower than the lab value, and the overall variability was notably higher than what you’d get from a standard lab draw.
That doesn’t make them useless for tracking trends between lab visits, but for an official diagnosis of diabetes or prediabetes, the American Diabetes Association recommends using only FDA-approved devices in certified testing facilities. If you’ve used a home kit and the number surprises you, a lab-based test is the way to confirm it.
When the Test Can Be Misleading
Because A1C depends on the normal life cycle of red blood cells, anything that disrupts that cycle can skew results. Conditions that shorten how long red blood cells survive, such as hemolytic anemia or recovery from significant blood loss, will push A1C falsely low. Your red blood cells simply haven’t been around long enough to accumulate glucose at a rate that reflects your actual blood sugar.
Sickle cell disease presents a particular challenge. The combination of anemia, faster red blood cell turnover, and frequent transfusions makes A1C unreliable as a long-term marker. For people with sickle cell trait or disease, doctors often turn to alternative markers like glycated albumin, which measures glucose attachment to a blood protein that isn’t affected by red blood cell lifespan.
Chronic kidney disease can also interfere. In patients on dialysis, A1C tends to underestimate blood sugar levels. Chemical changes to hemoglobin that occur with kidney failure can throw off certain testing methods, making glycated albumin a more dependable option here as well.
Pregnancy adds another wrinkle. In late pregnancy, iron deficiency can push A1C higher even in women without diabetes, making the number less useful as a standalone indicator during the third trimester.
How A1C Differs From a Glucose Test
A fasting glucose test captures a single snapshot of your blood sugar at the moment your blood is drawn. Skip a meal, eat unusually well for a day, or feel stressed during the visit, and the number shifts. A1C is far less sensitive to day-to-day fluctuations because it’s averaging roughly 90 days of blood sugar exposure. That’s why it’s become a primary tool for both diagnosing diabetes and tracking how well a management plan is working over time.
The practical difference for you is straightforward: a glucose test requires fasting for at least 8 hours, while an A1C draw can happen any time of day regardless of what you’ve eaten. If your doctor orders both on the same visit, the fasting requirement for the glucose test is the one that dictates your prep.